Device for the protection of electrical apparatus



June 1, 1965 E. ANDRICH 3,187,164

DEVICE FOR THE PROTECTION OF ELECTRICAL APPARATUS Filed Dec. 5, 1962 INVENTOR EKKEHAHJ ANDRICH AG NT United States Patent O M 3,187,164 DEVICE FOR THE PROTECTION OF ELECTRICAL APPARATUS Ekkehard Andrich, Aachen, Germany, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Dec. 5, 1962, Ser. No. 242,387 Claims priority, application Germany, Sept. 27, 1962, N 22,142 7 Claims. (Cl. 219-504) This invention relates to temperature control devices and more particularly to a temperature control device for preventing thermal overload in a body.

For protecting electical apparatus from thermal overload it is possible by means of a temperature-dependent element to release a control current which actuates a relay included in the circuit for feeding the apparatus, which relay switches ofi or reduces the supply current for the apparatus.

Such a solution is very expensive and it is an object of the present invention to provide a device which is much simpler and cheaper in this respect.

In a known device for the electrical heating of an apparatus with automatic temperature control, a resistor having a positive temperature coefficient is connected in series with the heating element of the apparatus. Said resistor fulfills the function of a current-limiting resistor and prevents a further rise in current as soon as the apparatus has reached a certain temperature. However, the operation of this device is not too reliable since variations in the supply voltage or in the heat dissipation of the apparatus bring about corresponding variations in temperature so that there is insufficient protection from a certain critical temperature being exceeded. Besides, more energy is dissipated in said current-limiting resistor upon increasing supply voltage so that a risk of overload on said resistor and hence inadmissible operation of the device is involved.

The present invention relates to a device for protecting an electrical apparatus from thermal overload in which a resistor with a positive temperature coefficient (PTC), which is thermally coupled to the subject to be protected, is included in series in the circuit for feeding supply energy to the electrical apparatus. Its object is to combine the reliability of the first-mentioned device and the simplicity of the second-mentioned device and is character ized in that the PTO-resistor has so high a temperature coefiicient and is in such thermal contact with the subject to be protected that upon reaching the critical temperature the operating current of the device is shifted to the negatively sloped part of the current-voltage characteristic of the PTC-resistor.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing, in which:

FIGURE 1 shows one embodiment and FIGURE 2 shows current-voltage characteristics.

FIGURE 1 shows a device for heating liquids, for example, for boiling water. It comprises a liquid container 1, a heating winding 2 and a series resistor 3 having a positive temperature coeflicient. Resistor 3 is connected to the supply mains 4 in series with the heating winding.

According to the invention, the resistor 3 has so high a temperature coefiicient and is arranged in such thermal contact with the wall of the container and with the liquid that its current-voltage characteristic (1 over V) is shown by the curve T in FIGURE 2 at low temperatures and by the curve T at high temperatures. A resistance element exhibiting the type of current versus voltage characteristic of the invention,

3,187,1ti4 Patented 'June 1, 1965 illustrated in FIG. 2, and suitable for use in this invention, is described in British Patent No. 714,965, published September 8, 1954. The supply voltage is indicated by V and the load line formed by the heating winding 2 is indicated by R. Since the load represented by heating element 2 remains substantially constant, load line R is a straight line which intercepts the 1 axis at the value of current which would flow in the series circuit for a given value V of the source voltage if resistor 3 were short circuited.

When the device is connected to the mains the resistor 3 shows the characteristic T and is adjusted to working-point P whereby a comparatively large current flows through the series-arrangement 23 and the voltage drop across resistor 3 is very low relative to the supply voltage V A considerable amount of heat is thus evolved in the heating winding 2 and the temperature rises. As a result, the I-V-characteristic of resistor 3 is shifted towards the curve T in FIGURE 2.

As soon as the points of intersection P and P of said characteristic coincide with the load line R, i.e. upon reaching the critical temperature, an instable condition arises and resistor 3 is adjusted to working-point P in the lower branch of the I-V-characteristic where the voltage drop across resistor 3 is substantially equal to the mains voltage V and the current is reduced in a stepwise manner to a lower value. Consequently, very little heating energy is then supplied to the container 1 so that the temperature cannot rise further.

The critical temperature at which the points of intersection P and P are coincident depends only very little upon the unavoidable variations in mains voltage or in the heat dissipation of the device. Upon increasing mains voltage V whereby more electrical energy is supplied to the heating winding 2 the operating characteristic R is shifted in such a manner that the points of intersections P and P approach each other. The heating thus becomes greater but the critical temperature at which the current flowing through the series-combination 23 declines is also reached sooner. If, however, the resistor 3 would show the usual current-saturation characteristic the device would be adjusted to an increased thermal output of the heating winding upon an increase in supply voltage and hence to a new thermal equilibrium at a higher temperature. The energy I X V dissipated in resistor 3 would then increase considerably since with constant current the voltage across resistor 3 would increase, which may result in a material risk of overload. Also the heating-up will take place more rapidly or more slowly if the heat radiation or other heat dissipation of the device varies, but the temperature at which the current declines will not substantially vary.

In a preferred solution, the resistor 3 is brought only into temporary thermal contact with the article to be guarded. In the device of FIGURE 1 the resistor 3 is arranged so as to be exposed to the vapour of the liquid present in the container 1. As soon as the liquid is boiling and a substantial flow of vapour streaks along said resistor, the thermal contact between resistor 3 and the liquid to be guarded will be considerably improved so that a very critical switching-off of the current by the series-combination 2, 3 is ensured.

As a further variant the heating winding 2 and the resistor 3 may be combined into one unit wherein the former itself has the high positive temperature coefficient required. Furthermore, the thermal contact between resistor 3 and the liquid to be guarded, or the container 1 may be varied by means of a bimetal strip (not shown) in order thus to obtain a commutation which is even more critical.

It will be evident that the device may also be used 9 in the heating of solid articles. It may also serve to protect an apparatus from thermal overload without elecrical heating taking place in the apparatus intentionally.

What is claimed is:

= 1. A temperature control device for protecting a body from thermal overload, comprising resistance means having a high positive temperature coefli-cient, and a current versus voltage characteristic curve having a first region with a positive slope and a second region having a negative slope, means for thermally coupling said resistance means with said body, an electric heating element for heating said body connected in series circuit with said resistance means, and input means for supplying electric energy to said series circuit thereby to heat said body to a given temperature, said resistance means having a first resistance value below said given temperature and a substantially greater resistance value Whenever the temperature of said body reaches said given temperature.

2. Apparatus as described in claim 1 in which said thermal coupling means provides a variable degree of thermal coupling between said body and said resistance means which is thermally dependent upon the temperature of said body, said thermal coupling between said body and .said resistance means increasing with the temperature of said body.

3. A temperature control device for protecting a body from thermal overload, comprising resistance means arranged in thermal coupling relationship with said body, said resistance means having a high positive temperature coefficient so that it exhibits a negative resistance region in its characteristic current versus voltage curve, an electric heating element for heating said body connected in series circuit with said resistance means, and input means for supplying electric energy to said series circuit thereby to heat said body to a given temperature, said resistance means having a first resistance value below said given temperature and a sharply increased resistance value at said given temperature of said body whereby said resistance means then operates in its negative resistance region.

4. A temperature control device for protecting a liquid substance held in a container from excessive heating, comprising a resistance element having a high positive temperature coefiicient and exhibiting a negative resistance region, an electric heating element for heating said liquid to a given temperature connected in series circuit with said resistance element, said liquid producing a vapour in response to the heating thereof, and input means for supplying electric energy to said series circuit thereby to supply a heating current to said heating element, said resistance element being positioned in said container over said liquid in the path of the vapour produced by heating said liquid thereby providing a variable thermal coupling between said liquid and said resistance element, said resistance element having a relatively low resistance value when said liquid is below said given temperature, said resistance value sharply increasing in response to the increased thermal coupling produced by said vapour whenever the temperature of said liquid attains said given temperature, whereby said resistance element operates in said negative resistance region.

5. Apparatus as described in claim 4 wherein said given temperature is the boiling point temperature of said liquid.

6. A temperature control device for protecting a body from thermal overload, comprising resistance means arranged in thermal coupling relationship with said body, said resistance means having a high positive temperature coefiicient and exhibiting a first region having a substantially linear current versus voltage characteristic and a second negative resistance region, an electric heating element for heating said body connected in series circuit with said resistance means, and input means supplying electric energy to said series circuit thereby to heat said body, said resistance means being adapted to operate in said first region below a given temperature of said body and abruptly shifting to said negative resistance region Whenever said body attain-s said given temperature thereby substantially reducing the heating current flowing in said series circuit.

'7. A temperature control device for protecting a liquid substance held in a container from excessive heating, comprising a resistance element having a high positive temperature coefficient and exhibiting a current versus voltage characteristic curve having a first region with a positive slope and a second region having a negative slope, an electric heating element for heating said liquid arranged in thermal coupling relationship thereto and connected in series circuit with said resistance element, and input means for supplying electric energyto said series circuit thereby to heat said liquid to its boiling point and produce a substantial amount of vapour, said resistance element being positioned adjacent said'liquid in the path of the vapour produced thereby to provide a variable degree of thermal coupling between said liquid and said resistance element, said resistance element having a" relatively low resistance value when said liquid is below its boiling point and sharply increasing in resistance in response to the increased thermal coupling provided by said vapour when said liquid reaches its boiling point, whereby said resistance element then operates in said negative slope region so as to limit current flow in said series circuit.

References Cited'by the Examiner UNITED STATES PATENTS 2,163,297 6/39 .Waage 219-494 2,896,056 7/59 Borrup et a1. 2l9--50=8 3,060,298 '10/62 Swanson 219-505 RICHARD M. WOOD, Primary Examiner. 

6. A TEMPERATURE CONTROL DEVICE FOR PROTECTING A BODY FROM THERMAL OVERLOAD, COMPRISING RESISTANCE MEANS ARRANGED IN THERMAL COUPLING RELATIONSHIP WITH SAID BODY, SAID RESISTANCE MEANS HAVING A HIGH POSITIVE TEMPERATURE COEFFICIENT AND EXHIBITING A FIRST REGION HAVING A SUBSTANTIALLY LINEAR CURRENT VERSUS VOLTAGE CHARACTERISTIC AND A SECOND NEGATIVE RESISTANCE REGION, AN ELECTRIC HEATING ELEMENT FOR HEATING SAID BODY CONNECTED IN SERIES CIRCUIT WITH SAID RESISTANCE MEANS, AND INPUT MEANS SUPPLYING ELECTRIC ENERGY TO SAID SERIES CIRCUIT THEREBY TO HEAT SAID BODY, SAID RESISTANCE MEANS BEING ADAPTED TO OPERATE IN SAID FIRST REGION BELOW A GIVEN TEMPERATURE OF SAID BODY AND ABRUPTLY SHIFTING TO SAID NEGATIVE RESISTANCE REGION WHENEVER SAID BODY ATTAINS SAID GIVEN TEMPERATURE THEREBY SUBSTANTIALLY REDUCING THE HEATING CURRENT FLOWING IN SAID SERIES CIRCUIT. 